Method for reducing compositional gradients in{11 {11 {11 {11 {11 {11 {11 {11 {11 {11

ABSTRACT

Compositional gradients in a body of Hg1 xCdxTe can be removed by annealing the body at a temperature which is greater than the solidus temperature and less than the liquidus temperature for the average composition of the body.

United States Patent [72] inventor [21 Appl. No. [22] Filed [45]Patented [73 Assignee 54 METHOD FOR REDUCING COMPOSITIONAL GRADIENTS INHg1 Cd Te [50] Field ofSearch 148/13, 13.1, 20.3, 1.6, 3

, [56] References Cited UNITED STATES PATENTS 3,468,727 9/1969 Gross148/13 3,537,912 11/1970 Aven et a1 148/1.6

Primary Examiner-Richard 0. Dean Attorneys-Lamont B. Koontz and Omund R.Dahle 2 Claims, 1 Drawing Fig.

8 ABSTRACT: Compositional gradients in a body of Hg; Cd, [52] U. Cl148/13, Te can be removed by annealing the body at a temperature 7 148/l which is greater than the solidus temperature and less than the [51 1Int. Cl C22 U16 liquidus temperature for the average composition f thebody TL}. L3 84 si S l T 4 53 TEMPE RATURE 0 X 5 X| X4 X 2 MOLE RATIO X,OF CdTo PATENTED 23 3.622.405

TE M PE RATURE MOLE RATIO, X, OF CdTe IN VliN'l ()R,

JOSEPH L. SCHMIT ATTORNEY.

METHOD ron REDUCING COMPOSITIONAL GRLIENTS IN H .-.cd.;re

BACKGROUND OF THE INVENTION The development of solid-state detectors ofwave lengths within the infrared portion of the electromagnetic spectrumhas led to the use of semiconductor alloys having the proper energy gapfor intrinsic photoconductivity at wave lengths within the range of 1.7to 30 microns. One successful intrinsic detector material that has beendeveloped for photoconductive detectors is mercury cadmium telluride, asemiconductor material which is an alloy of a semimetal, mercurytelluride, and a semiconductor, cadmium telluride. The mole ratio, x, ofcadmium telluride in the alloy determines the energy gap and thereforethe optical and semiconducting properties of the alloy. Since the energygap of mercury cadmium telluride is determined by the composition of thealloy, it is highly desirable to produce an ingot of mercury telluridehaving a uniform composition.

It has been extremely difficult to produce an ingot of Hg Cd Te havingaconstantcomposition. This difficulty arises from the tendency of thecompounds to segregate when cooling, giving different compositionalregions within the ingot that has been formed. The reason for thistendency to segregate can be explained graphically by a phase diagram,FIG. I, which has as its abscissa the composition, or mole ratio, of thematerial and as its ordinate the temperature.

The phase diagram forHgi-rcdzTe shows three regions, or phases: Theliquid phase, which is the region above the liquidus line 10, the solidphase, which is the region below the solidus line 11, and the liquidplus solid, or slush phase, the lens-shaped region between the liquidusand solidus lines. When a certain mole ratio, X of CdTe within theliquid phase is cooled to a temperature T located on the liquidus line,material having a mole ratio x is precipitated. This is caused by thefact that the liquidus and solidus lines are separate, and for a giventemperature T there is a point Ll on the liquidus line which correspondsto a mole ratio x, and a point S2 on the solidus line which correspondsto a mole ratio x Therefore, the first precipitated lig C d Te has amole ragg x As the CdTe rich material is precipitated out, the amountCdTe remaining in the liquid is depleted so as to relocate thecomposition of the liquid to a point at the left of point Ll. Uponfurther cooling, the liquid having composition x reaches point L3 and asolid is precipitated out of composition x For this reason an ingot of;ilg, ,CdITe exhibits compositional gradients over its length.

Growth of mercury cadmium telluride can be accomplished by the use ofthe modified Bridgman method described by E. L. Stelzer et al. in theIEEE Transactions on Electron Devices, pages 880-884, Oct. 1969. In thismethod stoichiometric amounts of the three elements, mercury, cadmium,and tellurium, plus some excess mercury are loaded into a thick wallquartz capsule, which is then evacuated and sealed off. The sealedcapsule is heated in a furnace and rocked back and forth to insuremixing, after which it is solidified from one end by sequentiallycooling the three zones of the furnace. Rapid solidification reducesthermal segregation, resulting in a largely single crystal ingotcontaining a dendritic structure, with altemating regions of high andlow mole ratios. As the cooling rate is increased, the dendriticstructure becomes finer. A subsequent high-temperature anneal, at atemperature below the solidus line is then used to remove the dendritesand a low temperature anneal is used to adjust stoichiometry. While itis possible to remove the dendritic microscopic compositional gradients,which are generally less than 1 mm., the high-temperature annealing steptakes weeks and leaves macroscopic x gradients on the order of severalmillimeters unaffected. Removal of macroscopic .r gradients by thismethod takes months or even yours and is therefore impractical.

A process known as zone leveling can be used to remove macroscopiccompositional gradients. Zone leveling entails heating a small region ofthe ingot to above the melting temperature, and then advancing themolten region, or zone through the length of the ingot at a rate whichis slow enough for thermal equilibrium to be approximately maintained.As the zone advances, the material left behind solidifies and newmaterial is melted. The first material to resolidify has a greaterconcentration of CdTe than the material which has yet to be melted. Asthe zone progresses, the concentration of the liquid changes until itattains a concentration which produces, upon solidification, materialwhich has a concentration which is equal to the concentration of theregion about to be melted. When this condition is attained, theconcentrations of the solids entering and leaving the zone are equal,and hence no further change of concentration occurs in the zone or inthe solid freezing from it until the zone reaches the end of the ingot.

The disadvantage of the zone-leveling process is that the movement ofthe zone is very slow, since the ingot must be in near thermalequilibrium. An additional disadvantage is that it is difficult tocontrol the liquid-solid interface and therefore compositional gradientsmay still occur.

SUMMARY OF THE INVENTION In the present invention, compositionalgradients in an ingot o f Hg ,Cd Te can be removed by annealing at atempeg ture which is greater than the solidus temperature and less thanthe liquidus temperature for the average composition of the ingot. Dueto the greatly reduced annealing time with this m ho tafieriC r oi nifpos on an A s produced by forming ingots quickly, removing thecompositional gradients by annealing the body at a temperature above thesolidus temperature and below the liquidus temperature for the averagecomposition of the body, further annealing the body at a temperaturenear but below the solidus temperature, and adjusting stoichiometry witha low-temperature anneal.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a (temperature, mole ratio)phase diagram for Hit-4 5 i- BRIEF DESCRIPTION OF THE PREFERREDEMBODIMENT In the present invention, an ingot HgI-ICdITe havingdendrites and compositional gradients is heated to a tempera ture abovethe solidus line and yet below the liquidus line for the averagecomposition x, of the ingot. This shifts the composition of theprecipitated material within the dendritic regions closer and closer tothe average composition x,. This process occurs at a much faster ratethan in a similar process employing heating below the solidus linebecause the material has a greater diffusibility when heated above thesolidus line. The greater diffusibility is due both to the highertemperature of the annealing and also to the fact that for a giventemperature T between the solidus and liquidus temperatures T and T ofaverage composition X that material having a smaller concentration ofCdTe than x is liquid, thus allowing it to flow and mix. Due to thishigher diffusibility and mixing, it is possible to remove macroscopiccompositional gradients as well as microscopic ones. By this method, onecan obtain an ingot of Hg1-ICdITe which is closer to the desireduniformity over greater distances in a much shorter time than has beenpossible by prior art techniques.

It can be seen that the higher the temperature is within the specifiedrange, the greater the difiusibility of the material is, and thereforeannealing time is at a minimum when the temperature is just below theliquidus temperature.

In one preparation, an ingot having a composition ranging from x=0.l30to x=0.3l8 over l3 mm. was annealed for 10 days at a temperature T=75SC., which is greater than the solidus temperature and less than theliquidus temperature for the average composition, M270. The resultantingot had a composition x=0.270i0.02 over the same distance.

3 4 A second ingot annealed at 750 C. for 8 days was found to propertyor right is claimed are defined as follows: have a composition x=0.561:0.0l4 across the diameter of 13 1. A method for reducingcompositional gradients in a body mm. of Hg, ,Cd Te wherein the body isannealed at a tempera- Due to the greatly reduced annealing timeobtained by this ture which is greater than the solidus temperature andless method Hg, ;Cd.rTe of uniform composition which i uit than theliquidus temperature for the average composition of ble for detectorscan be produced by forming ingots quickly, Said yremoving macroscopiccompositional gradients by annealing The method accordancc wlth clalm 1and further the body at a temperature which is greater than the solidusPflsmgi temperature and less than the liquidus temperature of theannefilmg the y at a temperature near h average composition of theingot, removing microscopic com- Sohdus temperature for the averageComposltlo" of said positional gradients with further annealing at atemperature lf and near but below the solidus temperature, and adjusting'f g the body at a lower temperature to adlust stoichiometry with alow-temperature anneal. stolchlometry' The embodiments of the inventionin which an exclusive l

2. The method in accordance with claim 1 and further comprising:annealing the body at a temperature near but below the solidustemperature for the average composition of said body, and annealing thebody at a lower temperature to adjust stoichiometry.